Hot Mix Asphalt, HMA, is a highly engineered product used in the paving of roads. For the purposes discussed herein, HMA consists of three components: 1) A bitumen mixture; 2) A stone aggregate; and 3) Air. The HMA is produced in a plant where a specified stone aggregate is mixed with a specified bitumen at temperatures up to 175 degrees Celsius (approximately 350 degrees Fahrenheit). The bitumen coats the aggregate. The mix is then sent to a paving site where the mix is spread with a paving machine (also called a paver or screed). When the HMA is first laid by the paver, its temperature may be around 135 degrees Celsius (approximately 275 degrees Fahrenheit) and contain around 15% air voids. In order for the HMA to meet design criteria, the air void content is commonly reduced to around 5%. This reduction is accomplished by rolling the HMA while it is in a temperature zone which permits it to be compacted. In some cases, the HMA is compacted using one or more types of rollers. A first type of roller is a break-down roller, or vibratory roller. FIG. 1 shows a highway grade steel drum vibratory roller according to the prior art. In some cases, this vibratory roller is the first roller used in the application process, and provides most of the compaction of the HMA. A vibratory roller conventionally has one or two steel drums with rotating weights, which vibrate the drums and create a dynamic force, adding to the dead weight of the roller, and increasing the compacting force (e.g., upon the HMA). A second type of roller conventionally used in the compaction of HMA is a rubber-tired roller, or pneumatic roller, which includes a number of rubber tires staggered to provide distributed compaction coverage. The rubber-tired roller is designed to knead the surface of the HMA and close the pores formed on the top surface of the asphalt. A third type of roller used in the compaction of HMA is a finish (or, finishing) roller, which is used last (e.g., after the vibratory and/or rubber-tired rollers) to minimize or eliminate creases formed by one of the previous rollers, leaving a smooth surface.
The quality of the paving is conventionally measured by the smoothness of the finish, as well as the density of the finished product. An under-compacted asphalt mat is permeable to air and water, which shortens the pavement life. In addition, in under-compacted asphalt, the presence of undesirable air voids may make the asphalt pavement less stable, as the number of inter-particle contact points is reduced and are more susceptible to freeze-thaw conditions. On the other hand, unnecessary over-compaction may crush the aggregate asphalt, leading to a reduction of air void content, which, in turn, can make the pavement susceptible to permanent deformation. If the pavement is under-compacted or over-compacted, the paving contractor may be penalized. It is therefore beneficial to achieve the desired level/range of compaction. Measurement of density may be accomplished during or after the rolling process by various hand-held instruments following corresponding ASTM International standards that provide a point measurement of density. Conventionally, after paving is completed, a core or plug about 15.25-cm (6-in) in diameter is cut from the pavement and its density is measured according to various ASTM International standards.
This approach of spot monitoring during and after rolling presents a number of problems. One issue is that very small areas of the roadway are subject to inspection. The typical commissioning authority (e.g., department of transportation or the like) requirement is to conduct one measurement on every 1,000 feet of road lane paved. This very limited sample is likely not a representative sample of the overall paving project. One earlier conventional approach attempting to address the issue of continuously measuring asphalt density from a roller device is described in U.S. Pat. No. 5,952,561 (which is hereby incorporated by reference in its entirety). This approach was supported by the US Department of Transportation (US DOT) Research Board's Innovations Deserving Exploratory Analysis (IDEA) program in 1997. The approach described in U.S. Pat. No. 5,952,561 was based on using microwave sensors mounted in the front and back of the roller device to detect asphalt densities (see PRIOR ART, FIG. 2). The actual wave lengths recommended for detection of asphalt densities in U.S. Pat. No. 5,952,561 were in the range of 1 to 3.75-cm (30 to 8 gigahertz). However, this approach has several shortcomings. For example, it has not yet been shown that microwave sensors operating from 500 megahertz to 30 gigahertz can provide an accurate density measurement of asphalt. Further, the measurement approach described in U.S. Pat. No. 5,952,561 was based on a difference between two readings, where the error band for each reading could exceed the value of the true reading. Thus, this conventional approach proved to be impractical.
A later conventional approach was based on the continuous measurement of the change between the input vibratory loads provided by the roller and the measured response of the material under the roller. This technique is called measuring the resilient modulus, or stiffness, of the pavement. The current nomenclature for this approach is called “Intelligent Compaction”, when these measurements are used to change the compactive effort delivered by the roller in a closed-loop fashion, and when combined with Global Positioning Satellites (GPS) and a Geographic Information System (GIS) mapping program.
This Intelligent Compaction approach is presented in U.S. Pat. Nos. 4,870,601, 5,727,900, 6,122,601, 6,551,019, 7,669,458, and 8,190,338 (each of which is hereby incorporated by reference in its entirety). This later conventional concept is illustrated in the prior art depiction of FIG. 3. In this approach, a vibratory impulse is imparted by the drum (or drums) of the roller into the asphalt, sub-base below the asphalt, and subgrade below the sub-base. This conventional approach assumes that the entire system can be modeled by a force being imposed on the various layers of materials, represented by a combination of springs and dampers. In the illustration in FIG. 3, there are three layers: the asphalt; the sub-base; and the sub-grade. The asphalt (or, pavement) layer is typically 25 to 30 cm (10 to 12 inches) thick, and the sub-base may have a similar thickness. The sub-base is typically an engineering specified grade of crushed rock. The sub-grade is typically the local soil that has been graded and compacted to provide the base for the road. Unlike the simplified illustration in FIG. 3, where there is a single layer of asphalt, in practice, there are multiple layers of asphalt. Within the asphalt, there is typically a base layer with a compacted thickness of about 13 cm (5.25-in), an intermediate layer with a compacted thickness of 9 cm (3.5-in) and a top or finish layer with a compacted thickness of 4.5 cm (1.75-in). The illustration in FIG. 3 is intended merely to depict paving of the base layer. As the number of asphalt layers increases and become thinner, the modeling becomes more difficult to relate the response of the individual fresh asphalt layer to the response of all of the material in the zone of influence of the vibrating roller, which is conventionally estimated to be from about 1.0 to 1.2 meters deep. This is one reason why vibratory-based Intelligent Compaction has not been very successful with asphalt pavement.
Some other problems facing vibratory-based measurement of asphalt are that the stiffness of the asphalt varies with its temperature. Cold asphalt is stiffer than warm asphalt, regardless of the actual amount of compaction. Additionally, the measurement is based on the roller drum which is around 220 cm (87-in) wide. This width of the roller may allow the roller to “bridge” over asphalt that is not being compacted.
A problem facing vibratory Intelligent Compaction is that there is presently no accepted way to convert the measured asphalt resilient modulus or stiffness into density. The actual density of the compacted asphalt is the parameter upon which engineering specifications are based.
As noted herein, the conventional approaches fail to provide a continuous quantitative determination of the density of the asphalt while it is being rolled.
The use of electromagnetic impedance measurement devices has been identified in U.S. Pat. Nos. 5,900,736, 6,414,497, and 7,219,024 (each of which is hereby incorporated by reference in its entirety) to provide a quantitative reading of the density of asphalt and other materials. However, the approaches disclosed in those patents require that the sensor remain in contact with the asphalt. In order to overcome this limitation, later approaches (as described in U.S. Pat. Nos. 7,226,239 and 7,575,395, each of which is hereby incorporated by reference in its entirety), were developed to use a then commercially available gauge in contact with the asphalt while mounted on a roller. This approach, however, was limiting in that it required the interruption of the normal roller operation to conduct measurements, and thus, was not continuous.